High tension separation of materials



y 30, 1967 R. 1.. BREAKIRON ETAL 3,322,275

HIGH TENSION SEPARATION OF MATERIALS Filed July 10, 1964 2 Sheets-Sheet1 I NVENTORS ATTORNEYS May 30, 967 R. 1.. BREAKIRON ETAL 3,322,275

HIGH TENSION SEPARATION OF MATERIALS 2 Sheets-Sheet 2 Filed July 10,1964 INVENTORS .w Z mr i 2 0. Wm Z 5 W f1? ATT RNEYS United StatesPatent 3,322,275 HIGH TENSION SEPARATION OF MATERIALS Robert L.Breakiron and William P. Dyrenforth, Jacksonville, Fla, assignors toCarpco Research & Engineering, Inc., Jacksonville, Fla, a corporation ofFlorida Filed July 10, 1964, Ser. No. 381,772 14 Claims. (Cl. 209-127)This invention relates to an improved method and apparatus for hightension separation and, more particularly, to an improved method andapparatus for the beneficiation of particulate materials containing alarge percentage of fines, ranging in size from about .150 microns toabout 5 microns.

High tension separation is an outgrowth of electrostatic separation butdiffers therefrom in many respects. The terms electrostatic implies thatno current is flowing. Basically, an electrostatic separation relies onthe principle that if particles in an electrostatic or external electricfield are differentially charged, particle trajectory will be soaffected that the desired fractions separately can be collected.Requisite differential charging normally is effected in the variouselectrostatic separation processes by contact electrification orinductive conduction or by a combination of both these phenomena.

In high tension separation on the other hand, the entire particulatefeed is sprayed with mobile ions, i.e., a corona discharge, while suchparticles are in contact with a grounded conductive surface such as ametal rotor. All of the particles are strongly charged by the mobileions. The non-conductors and poor conductors lose their charge slowly,are pinned to the ground conductive surface by their own image forcesand are suitably removed from the grounded conductive surface outsidethe locus of mobile ion charging. The conductive particles lose theircharges rapidly and, once removed from the influence of the mobile ionspray, are free to assume normal trajectories when discharged from thegrounded conductive surface under normal gravitational or centrifugalforces. If, outside the locus of mobile ion charging, the conductiveparticles now are subjected to an external electrical field either onthe grounded surface or in free fall, they again may be charged byinductive conduction in addition to contact electrification and theirtrajectories desirably altered to permit collection of various fractionsof the now essentially conductive particles.

A more detailed discussion of the phenomena encountered in conventionalelectrostatic and high tension separations can be found in Perry,Chemical Engineers Handbook, 4th edition, McGraw-Hill Pub. Co. (1966),section 21, pages 67 through 70, and in Lawver, Fundamentals ofElectrical Concentration of Minerals, The Mines Magazine, January 1960.

High tension separation currently is employed commercially t-obeneficiate a number of ores. In all cases, however, where high tensionseparation has been of significant success the particulate materialstreated have been characterized by a particle size of from -14 to +200mesh and have not contained a substantial amount of particles of a sizesmaller than 200 mesh. Attempts by the art to employ high tensionseparation with materials which necessitate grinding to an extremelyfine particle size in order to effect liberation, uniformly have beenunsuccessful. Unfortunately some of the most commercially significantores' today, such as hematite or other iron ores require fine grindingto effect liberation. Some iron ores, when comminuted to liberation,contain over seventy-five ercent of particles at -325 mesh and contain asubstantial amount of particles as small as five microns in diameter.The significance and economic advantages of an apparatus and methodpermitting the high tension separation or ores and other materialscontaining substantial quantities of dust-like particles ranging in sizefrom about 250 microns down to about 5 microns, readily is apparent.Numerous commercially significant areas previously foreclosed to anytype of dry beneficiation could then enjoy the advantages of this simpleand economical system if such method were available.

Therefore it is the primary object of this invention to provide animproved method and apparatus for high tension separation for materialscontaining substantial portions of fines.

It is the further object of this invention to provide an improved methodand apparatus for separating ores containing, when ground to liberation,substantial portions of fine material characterized by a particle sizeranging from about 250 microns down to about 5 microns.

An additional object of this invention is to provide a method forbeneficiation of ores wherein the particulate ore containing asubstantial quantity of dust-like fines successively is subjected to amobile ion discharge and an external electrical field, both of which arepulsating at a high frequency.

Other objects and advantages of the invention will become apparent fromthe general and specific description of this invention to follow.

The present invention is directed to a method and apparatus adapted tothe beneficiation of particulate materials and especially to theseparation of comminuted ores which are characterized by a sizablequantity of finely divided particles which will pass a 200 mesh screen.

Generally described the method of this invention is a method forseparation of particulate mixtures containing particles of varyingconductivity which comprises subjecting the mixture while disposed on agrounded conductive surface to a spray of mobile ions produced by acorona discharge pulsed at the rate of between about 150 to 800 pulsesper second, removing the mixture from the spray of mobile ions andseparately collecting the particles pinned to the grounded conductivesurface by the mobile ions and the particles not pinned to the groundedconductive surface by the mobile ions.

In terms of apparatus, the invention generally may be described as anapparatus for separating particulate materials containing particles ofvarying conductivity which comprises a grounded conductive surfaceadapted to convey the particulate mixture into a high tension chargingzone, a high tension charging zone defined by the grounded conductivesurface and a charging means positioned in spaced relation thereto foremitting a pulsed corona discharge into contact with the particlesdisposed on said surface at a pulse rate of between about 150 and 800pulses per second, external electric field generating means disposedoutside the high tension charging zone and in the direction of materialflow through the apparatus for subjecting at least part of theparticulate mixture to the action of an external electric field,collection means for separately collecting the particles pinned to thegrounded conductive surface by the corona discharge and collection meansfor the particles discharged from said surface through said externalelectric field.

It is preferred to employ a corona voltage of between about 10 kv. andabout k and a corona current strength of between 0.1 milliampere and 5.0milliamperes per foot width of grounded conductive surface.

In accordance with the preferred embodiment of the invention, thegrounded conductive surface will be a cylindrical rotor horizontallymounted to rotate about its longitudinal axis.

It is preferred that the corona discharge be unidirectional although itis not essential to obtaining the benefits of the invention that aunidirectional corona discharge be employed.

The present invention involves the concept that fine particulatematerials having particle sizes of 200 mesh and as small as, say,microns, may be separated or beneficiated in a high tension separationprocess where the corona discharge providing mobile ions for chargingpurposes is pulsed at a pulse rate of from about 150 to about 850 pulsesper second. The use of an external electrical field in conjunction withhigh tension separation in accordance with this invention is preferredto further improve the separation of the conductive particles of thefeed material which are not pinned to the grounded conductive surface bythe mobile ion discharge. In the external electrical field, whetherprovided by a free-fall apparatus or rotor and electrode equipment, theconductive particles recharge by inductive conduction and/ or contactelectrification and are deflected in accordance with this charge topermit more selective categorization. With some feed materials it isalso preferred that the external electric field be unidirectional.Preferably a field gradient of between about 20 and about 100 kv./ inchwill be employed. A beamed electrode directed away from the rotorsurface also may be employed in conjunction with an external electricfield in order to still further affect the trajectory of the conductiveparticles.

The polarity of both the corona and the electrodes creating the externalelectric field may be varied as dictated by the nature of the materialto be separated.

In conjunction with high tension separation wherein the corona dischargeis pulsed in accordance with the invention, it has been foundadditionally beneficial to employ rotor speeds faster than heretoforeused by the art. The combination of pulsed corona discharge, higherrotor speeds and external electrical field has been found to giveoptimum results with those materials such as hematite ores which whencomminuted to liberation contain as much as 75% by weight of particleswhich are 325 mesh.

Without being bound by any explanation of the surprising result of thisinvention, it is postulated that the success of the method and apparatusof this invention in the successful separation of particulate materialscontaining large proportions of dust-like materials may be explained bythe following considerations.

' The function of the corona discharge is to spray electrons or negativeor positive ions on to the feed particles fed to the rotor or othergrounded conductive surface. Charging, per se, of fine particles posesno problem. Indeed, owing to their relatively small surface area thesmaller particles actually require less charge to generate an attractionvalue than do the larger particles. However, under the conditionscreated by the continuous wave or 60 cycle half or full wave rectifiedpotentials heretofore used by the art in high tension separators,particles have all been strongly charged. Many of the large and smallparticles have thus arrived at the rotor at roughly the same time whilesubstantial portions of the smaller particles, and particularly those ofdust-like size, have arrived at a later time due to greater airresistance. As a result, the fine conductive particles have either beentrapped to the rotor surface by overlying larger pinned particles ofnon-conductors or else are last to arrive and lie over the pinned largernon-conductive particles and are unable to discharge.

It is believed that the process and apparatus of this invention obviatesthese conditions through two mechanisms. First the use of pulsed coronawithin the range of pulse rates disclosed enables the finer particles tobe charged before the coarser particles become fully charged and thusreach the rotor or other grounded conductive surface employed beforethey can be blocked out by the larger pinned non-conductors. This isespecially true where an increased rotor speed is employed in order thatthe finer particles are spread over a larger surface of groundedconductor surface. In accordance with the invention,

speeds of about 900 to about 1500 feet per minute have been foundeffective with rotors from 14 to 16 inches in diameter. As aconsequense, the conductive fines are discharged quickly and arerepelled for collection.

Secondly, the more conductive particles of the comminuted ores, such asiron ores, display ionic polarizability. The resulting ionic motionsthus create small particulate dipoles in the conductors. When thesesmall particulate dipoles are placed on top of the non-conductorsalready pinned to the rotor the dipoles will oscillate about theircenters of mass in the pulsed field of higher frequency employed in theinvention which causes the particles to shake loose from their boundcondition and reach a metal spot on the rotor where if they areconductors they will discharge and be rejected for collection. Thisphenomenon has been observed in high speed motion pictures taken duringoperation of the apparatus of this invention.

Having now generally described the method and apparatus of theinvention, a more specific description will be given with reference tothe drawings in which:

FIGURE 1 is a schematic end elevation of the separator of this inventionillustrating the flow pattern of minerals in this particular embodiment;

FIGURE 2 is a schematic end elevation of the separator of this inventionillustrating a typical space relationship for the various parts of theseparator; and

FIGURE 3 is a schematic perspective view of the separator of theinvention. I

Referring initially to FIGURE 1, a rotor 10, electrically connected toground 36, is mounted for revolution at varying speeds in the directionindicated by the arrow by means not shown. A feed hopper section 12 islocated above the rotor and terminates at a point above the upper faceof the rotor. The first discharge electrode 14 is spaced from the rotor10 with the electrode corona discharging wire portion 16 directedradially toward the rotor 10. A second electrode 20 is spaced from rotor10 beneath the first electrode 14. As shown in the embodiment of FIG-URE 1, electrode 20 is a dual purpose electrode which can be employedonly to produce an external electric field or as in the embodiment shownadditionally may be employed as a beam electrode. When the beamelectrode is employed the electrode wire 22 for corona discharge isdirected away from the rotor 10 for purposes hereinafter described inconjunction with the operation of the apparatus. A source ofunidirectional pulsed voltage 18 is connected to both electrode 14 andelectrode 20. Ind-ividual sources providing a different voltage for thetwo electrodes also may be employed.

A first splitter blade 26 is located beneath electrodes 14 and 20 and isspaced from the rotor 10. A second splitter blade 28 is located beneathrotor 10 and is spaced vertically therefrom. Three separate compartments46, 48 and 50 are located beneath the splitter blades for separatelycollecting the various fractions of the separated ore. A wiper wire 30connected to an alternating current source 32 is employed in theembodiment depicted and is positioned to the rear of and spaced from thesurface of rotor 10. A brush 34 is located above wire 30 and in contactwith the rotor 10.

In operation the mineral particles 38 are fed from the hopper 12 on tothe revolving rotor 10, preferably moving at a rate of from about 900 toabout 1500 feet per minute in the case of a rotor having a diameter offrom 14 to 16 inches. The rotor carries the ore on its surface past theelectrode 14 where the ore is charged by the pulsating corona discharge.The pulsed corona discharge causes the non-conductive particles to befirmly pinned to the surface of the rotor 10 while the conductiveparticles, including the fines, contact the rotor, lose their chargesand are repelled.

As the particles continue past electrode 20 the par ticles are subjectedto the effect of the external electric field produced by the electrode20 which in the embodiment shown in FIGURE 1 is powered at the samefrequency and voltage as electrode 14 by the power source 18. Conductiveparticles under the influence of the external electric field, as well ascentrifugal and gravitional forces, are removed from the surface of therotor and are displaced in the direction of electrode 20. In theembodiment shown in FIGURE 1 the corona discharge directed away from therotor 10 by wire 22 causes the trajectory of the particles to beaffected even further so that the majority of the conductive particlespass to the front of the first splitter blade 26 into compartment 50. Asthe rotor 10 continues its rotation the heavier conductive particleswith some of the gangue material fall by gravitational force betweensplitter blades 26 and 28 to form a middling fraction in compartment 48.As shown in FIGURE 1 essentially all of the pinned nonconduct-iveparticles are removed by the field generated by the electrode 30 andfall behind splitter blade 28 into compartment 46 as a tailing fraction.Any remaining, nonconductive material not removed by the wiper electrode30 is physically removed by the brush 34 and falls into compartment 46.

If desired, the middling material may be recirculated to exhaustion inorder to increase the recovery of valuable components of the ore.

For effective wiping, the AC. wiper should operate at high frequency,but at one which is not a harmonic of the pinning pulse. Thus, 300 or600 cycles has proved to be an effective wiping frequency to removematerial pinned by 400 pulses per second.

FIGURE 2 illustrates one spatial relationship which has proved effectivebetween the rotor 10 and electrodes 14 and 20. In FIGURE 2, the distancebetween the center of the rotor 10 and the center of electrode 14 isindicated by the letter A, and the distance between the center of therotor 10 and the center of electrode is indicated by the letter B. Theangle between the plane G-G passing through the horizontal axis of therotor 1t) and the radial center line passing between the center of rotor10 and the center of electrode 14 is indicated by the letter J.Similarly, the angle between the plane G-G of the horizontal axis ofrotor 10 and the radial center line passing between the center of rotor10 and the center of electrode 20 is indicated by the letter K. Theangle between the radial center line passing between the center of rotor10 and the center of electrode 20 and the center line passing throughthe center of electrode 20 and the electrode wire portion 22 isindicated by the letter L.

The proper location of the discharging electrodes is dependent upon thedecay time of the charges on the particles and upon the speed and sizeof the roto. Particularly good results in the separation of iron orefines have been obtained using a rotor about 14 inches in diameter,operating at from about 250 to about 400 rpm. With these conditionsdimension A should be about 10 /2 inches; dimension B should be about 8/2 inches; angle 1 should be about 45; angle K should be about 16; andangle L should be about 5.

With dust-like particles it is preferable to feed the ore at a pointforward of the vertical center of the rotor. This prevents overflowmaterial from building up and sliding down the back of the roller andalso prevents particles from receiving the pinning charge while stillabove the rotor surface. In FIGURE 2 the letter B represents thecircumferential distance from the vertical center FF of the rotor to thepoint where the hopper 12 terminates. Under the conditions specified inthe preceding paragraph, the distance E should be about 1% inches.

The following example is intended to illustrate the present inventionand is not intended to limit the invention beyond the limitations setforth in the claims.

Example Iron ore essentially all having a particle size of -325 mesh wasprocessed in accordance with the invention as herein described byfeeding the particulate mixture to a 14" mild steel rotor revolving at320 r.p.m. The two electrodes were operated at 400 pulses per secondusing negative unidirectional potential at 3 0 kv. The middling materialwas collected and recycled to exhaustion and various collections wereassayed with the results expressed in percentage of iron in eachcollection as follows:

It will be understood that the foregoing description, ex ample anddrawings are only illustrative of the present invention and it is notintended that the invention be limited thereto. Many modifications andvariations of the present invention may be made without departing fromthe scope and spirit of this disclosure and only such limitations shouldbe imposed as are indicated in the appended claims.

What is claimed is:

1. A method for effecting the separation of a particulate mixturecontaining particles of varying conductivity a substantial quantity ofwhich are of a mesh size of 200 which comprises subjecting the mixturewhile disposed on a grounded conductive surface to a spray of mobileions produced by a corona discharge pulsed at a rate of between about150 to about 800 pulses per second, removing the mixture from the sprayof mobile ions, and separately collecting the particles pinned to thegrounded conductive surface by the mobile ions and the particles notpinned to the grounded conductive surface by the mobile ions.

2. A method according to claim 1 in which the corona voltage is betweenabout 10 and about 100 kv. and the corona current is between about 0.1milliampere and about 5 milliamperes per foot width of groundedconductive surface.

3. A method according to claim 1 in which the grounded conductivesurface is curved and is rotated to carry the particulate mixture intoand out of the spray of mobile ions.

4. A method according to claim 1 in which the particles not pinned tothe grounded conductive surface are collected after first being passedthrough an external elec tric field.

5. A method according to claim 1 in which the particulate mixture is aniron ore comminuted to at least percent through 325 mesh.

6. A method for effecting the separation of a particulate mixturecontaining particles of varying conductivity a substantial quantity ofwhich are of a mesh size of -200 which comprises feeding such mixture tothe surface of a rotatable grounded conductive roll, passing the mixturewhile in contact with the roll through a spray of mobile ions producedby a corona discharge pulsed at a rate between about 150 and 800 pulsesper second, passing the particles not pinned to the roll by the mobileions through an external electrical field, collecting in at least onefraction the particles not pinned to the roll and passed through theexternal electric field, and separately collecting the particles pinnedto the roll by the mobile ions.

7. A method according to claim 6 in which the corona voltage is betweenabout 10 and about kv. and the corona current is between about 0.1milliampere and about 5 milliamperes per foot width of groundedconductive surface.

8. A method according to claim 6 in which the external electrical fieldalso is pulsed at a rate between about and 800 pulses per second.

9. A method according to claim 6 in which the particles pinned to theroll are removed therefrom at least in part by being subjected to afield generated by high frequency alternating current.

10. A method according to claim 6 in which the particulate mixture is aniron ore comminuted to at least 75 percent through 325 mesh.

11. An apparatus for separating particulate mixtures containingparticles of varying conductivity a substantial quantity of which are ofa mesh size of 200 which comprises a grounded conductive surface forconveying the mixture into a high tension charging zone, a high tensioncharging zone defined by the grounded conductive surface and a chargingmeans positioned in spaced relationship thereto for emitting a pulsedcorona discharge into contact with the particles disposed on saidsurface at a pulse rate of between about 150 and about 800 pulses persecond, external electric field generating means disposed outside thehigh tension charging zone and in the direction of material flow throughthe apparatus for subjecting at least part of the particulate mixture tothe action of an external electric field, and collection means forseparately collecting the particles pinned to the grounded conductivesurface by the corona discharge and the particles discharged from saidsurface through said external electric field.

12. An apparatus for separating particulate mixtures containingparticles of varying conductivity a substantial quantity of which are ofa mesh size of -200 which comprises a grounded conductive rotorhorizontally mounted to rotate about its longitudinal axis, means fordriving said rotor at varying speeds, feed means positionedsubstantially above said rotor for feeding the particulate mixture on tothe surface of the rotor, high tension charging means disposed adjacentthe surface of the rotor and in spaced relationship thereto for emittinga pulsed corona discharge into contact with the particles disposed onthe surface of said rotor at a pulse rate of about 150 to 800 pulses persecond, external electric field generating means disposed outside thehigh tension charging zone and in the direction of material flow throughthe apparatus for subjecting at least part of the particulate mixture tothe action of an external electric field, collection means forseparately collecting the particles pinned to the rotor surface by thecorona discharge and collection means for the particles discharged fromthe rotor surface through said external electric field.

13. Apparatus in accordance with claim 12 in which the collection meansfor collecting the particles pinned to the rotor by the corona dischargeincludes means for applying to said particles on the rotor, outside ofthe high tension charging zone and external electric field, an electricfield generated by a high frequency alternating current.

14. Apparatus in accordance with claim 12 in which the external electricfield generating means produces a pulsed field having a pulse rate offrom about 150 to about 800 pulses per second.

References Cited UNITED STATES PATENTS 805,694 11/1905 Wynne 209- X813,063 2/1906 Sutton 209130 X 3,031,079 4/1962 Boss 209128 FOREIGNPATENTS 681,171 3/1964 Canada.

OTHER REFERENCES Ralston, Electrostatic Separation of Mixed GranularSolids, 1961, TP 156, E5R3, C. 2; pp. 6466.

FRANK W. LUTTER, Primary Examiner.

1. A METHOD FOR EFFECTING THE SEPARATION OF A PARTICULATE MIXTURE CONTAINING PARTICLES OF VARYING CONDUCTIVITY A SUBSTANTIALLY QUANTITY OF WHICH ARE OF A MESH SIZE OF-200 WHICH COMPRISES SUBJECTING THE MIXTURE WHILE DISPOSED ON A GROUNDED CONDUCTIVE SURFACE TO A SPRAY OF MOBILE IONS PRODUCED BY A CORONA DISCHARGE PULSED AT A RATE OF BETWEEN ABOUT 150 TO ABOUT 800 PULSES PER SECOND, REMOVING THE MIXTURE FROM THE SPRAY OF MOBILE IONS, AND SEPARATELY COLLECTING THE PARTICLES PINNED TO THE GROUNDED CONDUCTIVE SURFACE BY THE MOBILE IONS AND THE PARTICLES NOT PINNED TO THE GROUNDED CONDUCTIVE SURFACE BY THE MOBILE IONS. 